In certain video disc systems, information is stored on a disc record in the form of geometric variations in a continuous spiral information track (e.g., a groove) disposed on the record surface. Variations in capacitance between an electrode incorporated in a groove-riding stylus and a conductive property of the record are sensed to reproduce the stored information. A capacitance-type video disc system is illustratively disclosed in U.S. Pat. No. 3,842,194 (Clemens).
In one format for the information track in practice of the Clemen's invention, depressed areas extending across the groove alternate with non-depressed areas, where the frequency of alternation varies with the amplitude of video information subject to recording. The form of the recorded signals may be, thus, a carrier whose frequency is modulated over a frequency deviation range in representation of video information. Certain practical considerations in the recording and playback processes have led to the location of the frequency deviation range at 4.3 to 6.3 MHz, and to the limitation of baseband video information to 3 MHz, for example. With this choice of parameters, the highest significant frequency component of the recorded signal is located at 9.3 MHz (e.g., 6.3+3.0 MHz).
In one electromechanical recording system described in U.S. Pat. No. 4,060,831 issued to J. B. Halter on Nov. 29, 1977, the cutterhead used to record the groove and signal information into a copper disc master or substrate is constructed such that its principle resonant frequency is established at a frequency in the midst of the frequency band where most of the energy of the encoded video signal components occurs. In this Halter system an equalizer network whose frequency response characteristic is complementary to the frequency response characteristic of the cutterhead is interposed between the signal source and the piezoelectric element of the cutterhead. The equalizer network serves two purposes. On the one hand, it cooperates with the cutterhead response characteristics to provide a frequency response characteristic which is relatively flat over the bandwidth occupied by the components of the encoded video signal. On the other hand, it serves to attenuate the energy level of signal components lying in the region where most of the energy of the encoded video signal occurs, thereby protecting the cutterhead from application of excessive electric fields. The attenuation of the applied energy makes possible the use of a smaller cutterhead than would otherwise be the case. The small cutterhead structure, in turn, results in a relatively wide cutterhead bandwidth.
The compensation necessary to provide a cutterhead which is flat over the bandwidth of the recording signal which, according to the Clemens patent, is up to 9.3 MHz may not be the same for cutterheads made under the same nominal conditions. Variations from device to device necessitate the matching of a given equalizer network to a given cutterhead. Therefore, a specialized equalizer must be provided for each particular cutterhead to effect a flat response. In the past the characteristics of a cutterhead have been ascertained by recording into a disc master and then measuring the groove and signal depth. This technique is undesirable since recording followed by measuring is a difficult and laborious process which may take hours to complete.